1. Field of the Invention
The present invention relates to a method for converting a binary data train, and more specifically to a binary data conversion method wherein binary data is recorded in a high-density recording medium, or binary data is transmitted through a transmission system having a relatively narrow bandwidth.
2. Description of the Prior Art
Various methods of binary data modulation record data onto a high-density recording medium, such as a magnetic tape, a magnetic disc, or an optical disc, or transmit binary data through a transmission system.
Such methods of binary data modulation, especially for the high density recording or the high density transmission, can be treated as combination of a process of binary data conversion and a process of modulation of a binary signal train which is obtained by the binary data conversion. In such a binary data conversion process, a binary data train is divided into a series of blocks, each having an M bit binary code (M is a natural number), and then, the M bit binary code is converted into an N bit binary code. The process of modulation is generally either an NRZI (Non-Return to Zero Inverse) fundamental modulation process or an NRZ (Non-Return to Zero) process.
Generally, the process of modulation for the high density of recording must comply with the following conditions:
(i) The minimum interval (referred to as T.sub.min hereinafter) of the inversion of the recording signal is sufficiently long and the maximum interval (referred to as T.sub.max hereinafter) thereof is sufficiently short;
(ii) The window of detection (referred to as T.sub.w hereinafter), which the time duration for detecting the recorded bits from a signal reproduced from the recording medium, is sufficiently wide; and
(iii) The signal to be recorded on the recording medium, which is obtained after modulation, does not contain a dc component or a low frequency component.
A long T.sub.min is suitable for reducing interference between two adjacent inversions and enables a high density recording. On the other hand, a short T.sub.max is advantageous for self-synchronization.
Since the detection of the recorded bits is effected by detecting peak values of a reproduced signal wave in the case of magnetic recording system, a wide T.sub.w, which determines the tolerance of the error of the position of detection, is advantageous for the high density recording. Also, in the case of a recording system employing a laser beam, the wide T.sub.w improves the signal to noise ratio due to an increased amplitude of the signal at the point of detection.
Furthermore, if a recording signal having a dc component or a low frequency component is applied to a transmission system which does not transmit the low frequency component, the waveform of such a signal is distorted. In addition, such a dc component or a low frequency component would cause a malfunction of the servo systems of a recording system employing a laser beam. If the signal does not include a significant and essential dc or low frequency component, a high pass filter can be employed to suppress the low frequency noise or drift components.
Among the prior art methods of modulation, the Zero Modulation (ZM) and the Modifield Miller (M.sup.2) techniques satisfy the above-descrived condition (iii); however, in those methods of modulation, the time length of T.sub.min is almost the same as the time length T of a bit of the data before modulation (T.sub.min .apprxeq.T in the case of the ZM, and T.sub.min =T in the case of M.sup.2), and therefore is not sufficiently long.
A long T.sub.min (T.sub.min =1.5T) is obtained in the Three Position Modulation (3M) technique, however, it does not satisfy the above condition (iii).
The Eight to Fourteen Modulation (EFM) technique which has been proposed recently, has a long T.sub.min of 1.41T, and satisfies the condition (iii); however, it has the drawback of short T.sub.w of 0.47T. Furthermore, the EFM technique has a problem that the structures or the modulator and the demodulator tend to be complicated since 8 bit data is converted to 14 bit data is a relatively large sized unit.